This invention relates generally to radar. More specifically, this invention relates to radar equipment which operates with a continuous or virtually continuous wave and which has a transmitter/receiver assembly capable of operating in two separate modes: namely, a first mode in which the carrier wave transmitted is frequency modulated for obtaining high resolving power with respect to range, and a second mode in which the carrier wave is pure, for obtaining high resolving power with respect to speed.
The invention relates particularly, but not exclusively, to a radar equipment which makes it possible to acquire and track an echo signal superimposed on undesired signals such as sea return signals, also known as clutter.
To detect by radar a target, such as a ship at sea, it is necessary to discriminate the target echo signal coming from from signals which are back-scattered by the surface of the sea. When the speed of movement of the target to be detected is low, or possibly zero, the process which provides adequate contrast between the echo signal and the clutter signals relies on the use of a waveform having a high resolving power with respect to range.
To achieve a high separating power with respect to range it is known to produce radar systems which operate with waveforms made up of pulses whose duration is extremely short, being of the order of one tenth of a microsecond (0.1 .mu.s) or less, representing a resolving ability with respect to range of approximately 15 m. In pulse radar of this kind, in order to achieve a high probability of detection, it is necessary for the peak power of the transmitted pulses to be high, on the order of tens of kilowatts or more. At the present time, in order to produce radar transmitters capable of providing such power levels reliance is placed on vacuum tubes, generally of the magnetron type, whose limitations and disadvantages are well known. In particular, tubes of this kind require a pulsed, high-voltage power source and a lengthly heating up period for the cathode. There is a condiderable reduction in the reliability of magnetron type vacuum tubes after long periods of storage or inactivity. In addition, it is difficult to produce a transmission/reception duplexer device, given the high level of microwave power and the short duration of the pulses. Furthermore, if military applications are considered, it is known that weapons systems fitted with radar equipment using pulsed operation are made vulnerable, or their effectiveness is considerably reduced, by the fact that radars of this kind are easily detected and located by monitoring receivers available to the enemy.
To overcome the above mentioned technical and operational problems which are inherent in radar equipment operating with short pulses, it has been proposed that pulses of relatively long duration, such as a hundred or more times longer than the duration of the short pulses, be transmitted and that the carrier frequency of these long-duration pulses be varied so as to achieve a high resolving power. Such a system known is pulse compression, is amply described in many publications. One such publication is chapter 20 of "The Radar Handbook", 1970 by M. L. Skolnik. Pulse compression techniques are divided into two major classes. The first such class relies on passive means and the second on active means. The invention herein relates particularly to second class.
Many problems and limitations become apparent when pulse compression techniques are practiced using active means. In effect, they must meet stringent conditions both in respect to the linearity of the frequency modulation of the carrier and with respect to the selectivity and distortion levels of the receiver circuits. These stringent requirements are further enhanced because the product of the frequency deviation F multiplied by the duration T of the pulses, which characterizes the transmitted signal, is considerable, being for example of the order of 10.sup.4 to 10.sup.5 or more.
Mention may also be made of a technique similar to pulse compression which relies on a frequency modulated carrier wave and which is made use of in radar sensors such as radioaltimeter units and proximity detectors. In equipment of this kind a precise measurement is required of the range at which a single object lies and resolving ability with respect to range is of subsidary or secondary importance.
The present invention alleviates the disadvantages inherent in prior-art radar techniques.